The present invention relates generally to photosensitive imaging compositions and more specifically to bi-layer photosensitive imaging films.
Probably the most commonly used photosensitive imaging systems employ silver halide based compositions. Silver halide contact films, for example, have been widely used for many years and are still the standard of the industry. Similarly, a wide variety of silver halide color proofing films and contact papers are in common use. Even printed circuit resists generally rely on silver halide based photosensitive systems.
Silver halide photosensitive imaging compositions are inconvenient to store and to use. They must be stored in light-tight containers. Their shelf life is very limited. Furthermore, processing must be carried out in the dark or under subdued, safelight conditions. Development of silver halide based compositions is generally a multi-step process requiring separate development, fixing and washing procedures. The chemicals utilized in these procedures are also ecologically undesirable, being oftentimes toxic and difficult to dispose of. Furthermore, even after processing, the silver halide based images deteriorate quickly and are readily scratched and otherwise susceptible to surface damage.
Silver halide films pose further difficulties when attempting to produce halftone images. For example, the production of halftone images of uniform optical density is quite difficult, particularly in those applications where it is further necessary to reduce image surface area by etching image dots and holes.
Silver halide imaging compositions offer very limited etching latitude. When the silver halide image is subjected to an etching solution, the solution attacks the perimeter of the image (dots in highlight areas, holes in shadow areas) and the top surface of the image as well. Thus, the thickness of the image is reduced along with its perimeter resulting in an image of reduced optical density. Since significant reductions in optical density are generally unacceptable in many applications (e.g. color proofing), only very limited etching of silver halide halftone images is possible. Furthermore, similar problems arise in non-silver halide based images where the image surface is unprotected from the etching solution.
Although alternatives to silver halide photosensitive imaging compositions have been proposed and are now offered in the marketplace, these alternative materials have not significantly displaced the silver halide materials for a number of reasons. For example, these alternative materials suffer many of the same disadvantages found in the silver halide materials, including storage difficulties, short shelf life, safelight and multi-step development, and ecologically undesirable processing procedures. These alternative compositions are also susceptible to scratching and other damage, both before exposure and development and after. As a result, post-development protective overcoats are often necessary. Furthermore, unlike the silver halide compositions, these alternative compositions often require multi-step exposures. Finally, and perhaps most importantly, these alternative materials usually have very limited development and exposure latitude and produce images of inadequate optical density and definition.